1. Field of the Invention
This invention relates to a numerical control error compensating system for compensating for an error which results from backlash, a pitch error, etc. of a machine placed under numerical control.
2. Description of the Prior Art
In the case of positioning by numerical control, it is customary to shift a movable part of a machine to a commanded position by the drive of a feed screw. When the feed screw is long, it is supported by bearings so as to be unmovable in its axial direction, for example, on the driving side but movable in the axial direction on the opposite side, whereby bending of the screw due to its expansion is prevented. Because of this feed screw bearing arrangement, positioning errors resulting from expansion of the feed screw differ greatly with feed direction; to avoid this, a high-diametered feed screw of large rigidity is used but such a feed screw is very expensive.
Furthermore, when the movable part is moved, by the feed screw, along its length of travel, a nonuniform distribution of pitch errors and an error distribution differing for each direction of feed due to the abovementioned expansion of the feed screw and so are often encountered; in particular, error curves in a forward and a backward feed do not bear a symmetrical relation to each other. If a machine, having such an error characteristic, is placed under numerical control, for example, when the forward movement to a commanded position is followed by the backward movement, then even if some backlash compensation is conducted, no complete error compensation can be achieved because of a difference in the amount of error at the position of reversal of the direction between the forward and backward movements. This difference is on the pitch errors therein etc. One cause of the change in the positioning error with the position of the movable part is that the load on the feed screw varies with the position of the movable part.